1. Field of the Invention
The present invention relates to a laser scanning unit for scanning information by using laser beams on or from a photo-sensitive drum in a laser printer, a facsimile unit, and the like. More particulary, the present invention relates to a laser beam deflecting device such as a digital micro mirror device which is disposed to be opposite to a photo-sensitive drum in a laser scanning unit, for deflecting laser beams toward the photo-sensitive drum.
2. Description of the Related Art
In general, a laser scanning unit such as one used in a laser printer, includes a laser diode, polygon mirror, and a photo-sensitive drum, and scans information recorded on a hologram disc, by using laser beams.
In the laser scanning unit, increasing the recording speed is achieved by increasing the deflection speed of the laser beams relating to each surface of the polygon mirror which is rotating.
The laser scanning unit scanning the information by using the laser beams forms images in dots on the photo-sensitive drum or reads images from an original draft. The laser scanning unit generally includes a laser diode for emitting laser beams, a laser beam deflecting device for deflecting the laser beams toward the photo-sensitive drum, and lenses for focusing the laser beams on the photo-sensitive drum.
FIG. 1 is a perspective view of a conventional laser scanning unit. The laser scanning unit according to the conventional art comprises a laser diode 100 for emitting laser beams, a collimate lens 101 for reforming the laser beams emitted from laser diode into parallel beams, a cylindrical lens 102 for reforming the parallel beams passing through the collimate lens 101 into linear beams, a polygon mirror 103 for deflecting the linear beams passing through the cylindrical lens 102 in constant velocity, a scanning motor 104 for rotating the polygon mirror 103 in constant velocity, image forming lenses 105 for focusing the laser beams deflected by the polygon mirror 103 on a photo-sensitive drum 107, a reflecting mirror 106 for reflecting the laser beams passing through the image forming lenses 105 and forming image on a surface of the photo-sensitive drum 107 with dots, a horizontal mirror 108 for reflecting a part of laser beams toward a photo detector 109, and the photo detector 109 for receiving a part of laser beams reflected by the horizontal mirror 108 and detecting errors.
The image forming lenses 105 includes a spherical lens 105a for correcting a spherical aberration and collecting the laser beams reflected by the polygon mirror 103 at constant velocity, and a toric lens 105b for focusing the laser beams of which the spherical aberration is corrected by the spherical lens 105a on the photo-sensitive drum 107.
In the laser scanning unit as constructed above, the laser diode 100 emits the laser beams. The laser beams emitted from the laser diode 100 are transformed by the collimate lens 101 into parallel beams.
The parallel beams passing through the collimate lens 101 are transformed by the cylindrical lens 102 into linear beams.
The linear beams passing through the cylindrical lens 102 are reflected by the polygon mirror 103 which is a beam reflecting element and which is mounted on a rotating shaft of the scanning motor 104. Since the polygon mirror 103 is rotated by the scanning motor 104, angles of the mirror surfaces of the polygon mirror 103 relating to the laser beams are changed. Thereby, the laser beams are deflected toward the image forming lenses 105 at the constant velocity.
The image forming lenses 105 correct the spherical aberration of the laser beams which are deflected by the polygon mirror at the constant velocity and focus the laser beams on the photo-sensitive drum. The image forming lenses 105 include the spherical lens 105a for correcting the spherical aberration of the laser beams and the toric lens 105b for focusing the laser beams on the photo-sensitive drum 107.
Therefore, the laser beams passing through the image forming lenses 105 are reflected by the reflecting mirror 106 so as to form an image on the surface of the photo-sensitive drum 107 with dots.
In the laser scanning unit of the conventional art, however, there is a problem in that since the surface of the polygon mirror is flat and inclined, it is very difficult to manufacture the polygon mirror used for deflecting the laser beams.
Also, there is another problem in that since the polygon mirror is rotated at high velocity to print the image at high velocity, the polygon mirror is subject to creating vibration and noise so that a printer is limited in printing speed and quality of image.
Furthermore, there is still another problem in that since an air bearing, a magnetic bearing, or a ball bearing is applied to the scanning motor to increase the rotating velocity of the polygon mirror, the size of the scanning motor increases.